Adding a Saniflo to existing soil stack

Old 06-17-19, 06:17 PM
Thread Starter
Join Date: Jun 2019
Location: United States
Posts: 1
Likes: 0
Received 0 Likes on 0 Posts
Adding a Saniflo to existing soil stack

I’m installing a Sanibest Pro grinding pump into a new bathroom I’m installing on the other side of the house.
The pump has 150ft range, and discharges waste over 1.5” PVC – so there’s plenty of range.

The closest (and easiest to access) waste stack is the master bathroom, and I thought I’d run the new waste through the attic and down the wall behind the toilet.

I assumed it would be a 3” or 4” cast iron waste stack, extending up into the roof as a vent.
I pulled off the wall behind the toilet and found all of that and more 😊!AiG1XjXoMk7YjrMPgHTzmnbkU58XuA

The 4” soil pipe appears to have a 4” to 3” transition about 6” long (made from cast iron). It then has a 3” to 2” transition about 3” long (made from copper, as far as I can tell)… which converts immediately to 2” copper which extends up through the roof as a vent stack. Both transitions appear to use old school oakum & lead poured seals. The whole 4” flange had also been covered in mortar which has now been cleared off.

(ignore the ½” copper at the back, running out to the toilet behind the wall – it looks like a small section of ½” copper has been soldered on to a brass 90` bend to shore it up against the vent stack to prevent movement because there was nothing else to fix it to)

Now, my question is this:
Should I remove the 3” cast iron section from the 4” waste stack, and insert a length of 4” ABS?
Should I remove the copper and install a length of 3” ABS leaving the 4"->3" transition

Would the city plumbing inspector have any concerns over leaving the old 3” to 4” cast iron transition?


So, the bloke in the plumbing shop said this will be a no-go with the inspector because wet-vents are prohibited in King County/Seattle. I don't understand why this addition makes the soil stack a wet vent...

Here's a rough diagram

The drawing on the left is the existing setup… on the right is what I wanted to do.

The Sanibest pump is independently vented to a different vent stack elsewhere in the house (closer)

I planned on taking out the 3” to 2” transition at the top of the cast iron soil stack, and inserting a new section of 3” ABS. On to that, I had planned on installing a 3” 45 degree WYE (with a 1.5” bush adapter) and run the discharge into that.

I don’t understand why this would make it a wet-stack – I’m not circumventing any traps, vents, etc… this is just the equivalent of having an upstairs and downstairs toilet on a single soil stack, but the dude was adamant.
Old 06-18-19, 05:34 AM
Group Moderator
Join Date: Mar 2003
Location: NC, USA
Posts: 28,348
Received 1,629 Likes on 1,462 Posts
Your far away toilet and pump that has a vent has no bearing on the venting situation where your pump is discharging. It has to do with the vent where you want to discharge. Wet venting between floors is not permitted. Discharging a toilet (via your pump) into the vent stack above another toilet is being considered a two story wet vent.
Old 06-18-19, 10:43 AM
Forum Topic Moderator
Join Date: Sep 2005
Location: USA
Posts: 6,364
Received 341 Likes on 307 Posts
Discharging a toilet (via your pump) into the vent stack above another toilet is being considered a two story wet vent.
Agreed with Pilot Dane, the way you have it laid out could suck the toilet trap dry when the saniflow flushes.

Two story bathrooms will (should) always have a separate venting from the downstairs bath.

Is there any way to tie the Saniflow into your main lateral in the basement?

Thread Tools
Search this Thread
Ask a Question
Question Title:
Your question will be posted in: